The invention refers to a device for measuring the position of a conductor in an enveloped cable.
In making cables the envelope is regularly extruded onto the conductor. The thickness of the envelope should have a predetermined dimension, to obtain proper insulation for example. Optical devices are provided to control the thickness or, respectively the diameter of the cable. A light source emitting light in a direction normal to the cable direction generates a shadow on a receiving device, for example on lines of diodes which is a measure for the diameter. When the diameter is not within the desired value, the speed of the extruder screw or the absolute speed of the conductor is adjusted until the desired value is obtained.
With a view to minimum material consumption, a smallest possible diameter is adjusted. However, when the conductor in the cable envelope is eccentrically offset, the desired wall thickness may fall short and the necessary insulation is not obtained anymore. Accordingly the eccentricity of the conductor should be controlled. When the eccentricity is measured at the end of the enveloping station, the correction by adjusting the extruder tools is relatively late; this means that a relatively long length of cable has an eccentricity out of allowance.
U.S. Pat. No. 4,086,044 discloses induction measuring means to determine the eccentricity. By means of inductive sensors the position of the conductor relative to the sensors is determined. The sensors are sensitive to the magnetic field for example which is generated by a current impressed in the conductor. The position of the cable envelope is determined by means of optical devices. In this way the wall thickness of the envelope is measured or calculated. However, the measurement of the eccentricity by means of inductive sensors including coils can hardly be performed in a pressure tube provided for plastic interlacing. In particular, an expensive temperature compensation is necessary to obtain approximately accurate dimensions.
The method referred to teaches a contactless measurement of the eccentricity. Moreover, methods of the engaging type are known as well. According to an embodiment of the known method a supporting member for a sensor contacts the cable envelope. The sensor includes an oscillator which dampening varies according to the distance of the conductor. It is further known to engage the cable with a pair of opposite plates of electrically conductive material to measure the capacity between the conductor and the plates, wherein a capacity difference different from zero indicates an eccentricity.
Both methods of the prior art exhibit substantial disadvantages. The supporting member or, respectively the electrically conducting plates are stationarily mounted, while the cable passing speed is substantial. For example, cables of small diameters pass through the extruder with a speed up to 2000 m/min. Accordingly, there is a substantial friction wear on the measuring head. Furthermore, there is a substantial contamination which is detrimental to the measuring accuracy. A further disadvantage of the known methods is the condition that each different diameter of the cable needs a new calibration. Still further, the known methods can be applied to the cable envelope only when fully cooled down. The method of measuring the capacitance has a further disadvantage that it suffers or becomes impossible when the envelope surface becomes hot in a humid environment.